1. Technical Field
This disclosure relates to a heat-sensitive adhesive agent that can generate adhesive ability through heating from room temperature, at which having substantially no adhesive ability, and can maintain the adhesive ability even after generating thereof; and a heat-sensitive adhesive sheet that contains a layer of the heat-sensitive adhesive agent on a support and can exhibit excellent adhesive strength to adherends.
2. Description of the Related Art
Adhesive label sheets have been increasing their application for price-display labels, article-display or bar-code labels, quality-indication labels, mass-indication labels or stickers. Processes for recording on labels have also been developed, such as ink-jet recording processes, heat-sensitive recording processes and pressure-sensitive recording processes.
These adhesive label sheets typically have a construction that an adhesive layer and a peeling paper are laminated on the surface opposite to information-recording surface of labels, and have been widely used since the sheets can be conveniently laminated by way of simply removing the peeling paper and pressing them when laminating. The adhesive label sheets are typically applied after removing the peeling paper; however, the peeling papers are discarded in almost all cases since it is difficult to collect and recycle the removed peeling papers. Accordingly, heat-sensitive label sheets with a heat-sensitive adhesive layer have been attracting attention in recent years that exhibit substantially no adhesive ability at room temperature and thus require no peeling paper (Japanese Utility Model Application Laid-Open No. 06-25869).
The heat-sensitive adhesive layer in such heat-sensitive adhesive label sheets contains a thermoplastic resin, a heat-meltable substance, and an optional tackifier (Adhesion Handbook, 12th edition, pp. 131-135, 1980, by Kobunshi Kankoukai Ltd.). However, such a heat-sensitive adhesive layer suffers from various problems including decrease of adhesive strength with time after generating the adhesive ability, lower adhesive strength to coarse adherends such as cardboard, or bonding of the heat-sensitive adhesive layer to its backside during storage period in a rolled condition. The adhesive strength to adherends and the blocking resistance typically conflict each other, that is, since increasing the adhesive strength tends to degrade the blocking resistance and improving the blocking resistance tends to lower the adhesive strength to adherends, simultaneous pursuit of the two properties is remarkably difficult and has not been achieved currently.
Ester compounds such as dicyclohexyl phthalate, for example, have been investigated heretofore as the heat-meltable substance in order to improve the adhesive property (see Japanese Patent Application Laid-Open (JP-A) Nos. 61-9479 and 07-278521). The inclusion of the heat-meltable substance tends to enhance the adhesive property to adherends; however, it is impossible to maintain stably the adhesive strength to coarse adherends such as cardboard, and also blocking resistance is considerable poor such that sticking tends to occur during storage period in a rolled condition.
Heat-sensitive adhesive agents containing a phosphorus compound and various sheets produced therefrom are also proposed, which are allegedly excellent in adhesive property to adherends and also exhibit excellent blocking resistance (see JP-A Nos. 2000-103969, 2000-191922, 2000-212527, 2004-117941, 2001-64401, 2001-262117, 2002-88678, 2002-338935, and 2004-117941). Among these, JP-A No. 2000-103969 proposes that a phosphorus compound having a melting point of 85° C. to 100° C. is utilized as the heat-meltable compound or solid plasticizer, thereby a material having a lower melting point can advantageously generate adhesive strength upon heating and the blocking resistance can be excellent. However, such a condition as 140° C. for 30 seconds is also necessary to generate the adhesive property in this proposal, therefore, it will be difficult to generate the adhesive ability by the energy from thermal print heads. That is, the insufficient generation of adhesive strength is practically compensated by the heating or activating condition rather than both of the adhesive strength and the blocking resistance are satisfied at higher level; it is believed that the heat-sensitive adhesive agent is currently insufficient in practical applications in order to be effectively utilized as various media.
It is also publicly known that various fillers may be incorporated as an adhesive agent in addition to the heat-sensitive adhesive agent in order to improve the blocking resistance, which being another object. The Patent Literatures described above disclose addition of conventional inorganic or organic particles as an antiblocking agent into the layer of heat-sensitive adhesive agent.
Specifically, as regards the construction that a layer of heat-sensitive adhesive agent and a layer of thermoplastic resin are disposed on a support, a method is proposed to incorporate a pigment into the layer of thermoplastic resin (see JP-A No. 11-279495), and a method is proposed to incorporate an oil-absorptive pigment (oil absorption number: 100 mL/100 g) into the layer of heat-sensitive adhesive agent that mainly contains a thermoplastic resin and a solid plasticizer (see JP-A No. 09-235528). However, both of the methods incorporate the pigment, which disturbing adhesion, into the outermost layer, therefore, the adhesive strength to adherends is lowered, that is, these methods are far from simultaneous pursuit of the blocking resistance and the adhesive strength and thus are impractical. In addition, a method is proposed to compound a lubricant wax into a heat-sensitive adhesive agent (see Japanese Patent Application Publication No. 62-21835), a method is proposed to protect the surface of the heat-meltable substance by means of inorganic compounds or colloid particles (see JP-A Nos. 06-57223, 06-100847 and 06-100848), and a method is proposed to use a filler having a porous crystalline structure as an inorganic filler in the layer of heat-sensitive adhesive agent (see JP-A No. 2002-114953). However, these proposals also tend to lower the adhesive strength to adherends and to be far from simultaneous pursuit of the blocking resistance and the adhesive strength.